In coal and other kinds of mining by the longwall technique, it is conventional to provide steel roof supports either as components of a series of hydraulically powered roof supports extending along, and advanceable with, the mineral face as the latter is extracted, or permanent supports in the form of rings or arches at roadways that are conventionally provided at each end of a longwall face.
Mineral is removed by a single or double ended ranging drum shearer, which traverses the mineral face, with a rotary cutting head carried by the, or each, ranging arm to follow the seam, each drum being provided with 50-100 cutter tools, each having a shank by which each tool is replaceably located in a tool holder.
However, despite the best efforts of the operatives, the drums, and specifically the tools, frequently strike a steel roof bar, or hard inclusions in the roof or floor of the mineral seam, and the tool is fractured—typically its head or blade, is sheared from its shank.
Tools need regular replacement, and removal of a broken or damaged tool is frequently difficult or sometimes impossible, such that removal cannot be effected until the drum is eventually taken to the surface, or a safe underground area, for refurbishment.
In civil engineering works, the tool may strike an unknown buried obstruction such as concrete reinforcement bar, a steel railway line, etc.